The present invention relates to single crystal semiconductors sensitive to electromagnetic radiation. More particularly, the present invention involves the geometry of photovoltaic detectors.
It has been found that the maximum voltage obtainable from a semiconductor device containing a single P-N junction is always limited to the width of the semiconductor bandgap or energy gap. The bandgap is the forbidden region of the electron energy spectrum in the semiconductor, and is equivalent to the energy range between the bottom of the conduction band and the top of the valence band. The conduction band is the range of states in the energy spectrum of a solid in which electrons can move freely, while the valence band is the range of energy states in a solid crystal in which electron vacancies, or "holes", can freely move.
It is known that a large class of semiconductor devices are photovoltaic. Such devices contain a rectifying barrier or a P-N junction at the interface or boundary between regions of the semiconductor having different electrical properties. In a P-N junction the boundary is a transition region between P-type and N-type conductivity semiconductor region materials. N-type conductivity semiconductor materials are those in which the charge carriers of electrical current are principally negative electrons. P-type conductivity semiconductor materials are those in which the charge carriers of electrical current are primarily electron-deficiency centers which act as positive holes. When a device of this class is exposed to light, it develops a voltage across the terminals of the unit. The term "light" is used in this application as not limited to the visible spectrum of electromagnetic radiation, but in the broad sense of radiant energy. Such devices do not require any external power supply nor, in a single P-N junction device, biasing of the junction, and are known as photovoltaic cells. Cells prepared with multiple, back-to-back junctions however, require an external power supply in order to bias alternate junctions.
A photovoltaic detector, typically a junction device, converts energy from impingent electromagnetic radiation into a photocurrent or a no-load potential difference. Discrete quanta of radiation with energy E.sub..lambda., enter the thin layer adjacent to the surface of the detector with sufficient energy (i.e., E.sub..lambda. .gtoreq.E.sub.g) to liberate an electron-hole pair. As these charge carriers diffuse to the potential gradient of the P-N junction, the majority charge carriers are repelled by, and the minority charge carriers are attracted to and swept across the junction thus generating a flow of electric charges. If the light reaches the semiconductor at a distance greater than the minority carrier diffusion length L from the rectifying barrier, the liberated carriers tend to combine before they reach the barrier and therefore do not contribute to the photocurrent. If the junction is externally shorted, a photocurrent flows, reducing minority carrier densities on each side of the junction to their equilibrium values. If the junction is not stored, a photovoltage is generated across the junction.
A photovoltaic detector may be characterized by three basic parameters: the spectral range over which it responds, the speed with which it responds, and the smallest radiant power it can detet. The spectral range is a function of E.sub.g of the semiconductor, which is dependent upon the composition from which the detector is fabricated. The response speed is limited by the more dominant of either the effective lifetime of the photoexcited minority carriers or by circuit characteristics, particularly the RC time constant .tau..sub.c, where R is the circuit resistance and C is the junction capacitance. In contemporary detectors carrier lifetimes vary between 10.sup.-9 and 10.sup.-8 seconds while the RC time constant is typically in excess of 10.sup.-6 seconds. In recent years, the centuries old impetus to increase the capacity of communication systems has focused upon a reduction in the time required to handle each unit of information. Photoemissive devices capable of generating signals with very short pulse durations, perhaps on the order of nanoseconds per pulse, are now available. Compatible ancillary analogue and digital signal processing circuits (e.g. preamplifiers) have long been available. The long electrical response time of photovoltaic detectors provided by the present art has forestalled construction of a photo-electric communication system utilizing the quick response time of recently available photoemissive devices, thereby denying a further enhancement of network capacity to the communication industry.
The third basic parameter, the smallest radiant power detectible by a photovoltaic device, is generally indicated by P.sub.N,.lambda., the noise equivalent power. Its reciprocal, normalized to an effective optical area of one square centimeter and a spectral response bandwidth of one Hertz, is the detectivity, D.sub..lambda. *. Detectivity is directly proportional to the quantum or external efficiency, .eta., of the detector, a value which is in turn proportional to the collection or internal efficiency, .eta..sub.coll. The quantum efficiency is defined as the ratio of the number of electrons crossing the junction to the number of photons incident upon the effective optical area of the detector. The collection efficiency is defined as the ratio of charge-carrier pairs separated by the electric field of the P-N junction to the total number of pairs generated. Both efficiencies are enhanced by a detector geometry providing for an increase in the number of electron-hole pairs generated within a diffusion length of the junction. Those not familiar with collection efficiency will find greater detail in Limitations And Possibilities For Improvement of Photovoltaic Solar Energy Converters, by M. Wolf, and published in Volume 48 of the Proceedings Of The IRE in 1960. The diffusion length is the average distance that a charge carrier diffuses between generation and recombination; on the average only those charge carrier pairs within a diffusion length of the junction will be collected. Photovoltaic detectors provided by the present art, such as that taught by Daniel Amingual, et alii, in U.S. Pat. No. 3,904,879, for example, depend upon electrical contacts that are separated from the junction by one or more sections of the detector layer, thereby increasing the distance that a minority charge carrier must travel in order to contribute to the current flow, and thus, the probability of recombination before reaching the contact. The quantum efficiency, and thus the detectivity of the present art thin-film devices is limited by their geometry as the operative surfaces of their electrical contacts are unnecessarily remote from the junction.